This is great, I've had this bookmarked for absolute ages. I think this should have been blogged as one of Craftster's suggested DIY christmas gifts because I completely forgot about this project and it would have been a great christmas gift.

Anyways I have a car now so I am going to make one today. First I just wanted to note that the formula the author is talking about is just a simple math formula to find circumfrence. Should be pretty easy:

C= Pi x Diameter

So the diameter of your circle (the length across the middle of the circle), if you are using a 9 inch pie plate would be 9 and Pi can be entered in your calc. as 3.14 (if you dont have the pi key). So you would cut a piece of fabric approximately 28 inches long plus seam allowances.

So in case you wanted to go for a bigger or a smaller circle you can find the length of fabric you need.